If four length values are specified they apply to top, right, bottom and left respectively. If there is only one value, it applies to all sides, if there are two or three, the missing values are taken from the opposite side.

This is the ante or deposit that is required in order to trade a futures contract. Initial margin is the way in which the exchange protects itself against bad debts relating to the futures contract, usually defaulting on variation margin payments (see below). Initial margins are calculated based on the current selling price and the market risk.
The initial margin is refunded at the time the futures contract expires.

Variation Margin

A futures contract can be to buy x or to sell x at some future date. Let us say, you have undertaken to buy x - i.e. you are betting that the price of x will rise. If the price moves in your favour, you receive a variation margin payment from the exchange. If the price falls, you are expected to pay a variation margin to the exchange, and to settle in 24 hours (T+1 settlement). This is called a margin call.

This process of adjusting for price movements happens on a daily basis, and is called marking to market.
The price may well be volatile, and there may be many payments to and fro for a given futures contract.

Specifically: The part of a page at the edge left uncovered in writing or printing.

3. Com.

The difference between the cost and the selling price of an article.

4.

Something allowed, or reserved, for that which can not be foreseen or known with certainty.

5. Brokerage

Collateral security deposited with a broker to secure him from loss on contracts entered into by him on behalf of his principial, as in the speculative buying and selling of stocks, wheat, etc.

N. Biddle.

Margin draft Masonry, a smooth cut margin on the face of hammer-dressed ashlar, adjacent to the joints. -- Margin of a courseArch., that part of a course, as of slates or shingles, which is not covered by the course immediately above it. See 2d Gauge.